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Polynucleotide

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Definition

noun

plural: polynucleotides

pol·y·nu·cle·o·tide, [ˌpɒlɪˈno͞o′klē-ə-tīd]


A biopolymer comprised of a long, linear series of nucleotides joined together by ester linkages between the phosphoryl group of nucleotide and the hydroxyl group of the sugar component of the next nucleotide




Details

Overview

A nucleotide is regarded as the basic building block of nucleic acid polymers (e.g. DNA and RNA). It is an organic compound made up of three subunits: a nucleobase (either a purine or a pyrimidine), a five-carbon sugar (pentose), and a phosphate group. The sugar component may either be ribose or deoxyribose. The ribose sugar is the sugar component of the nucleotides that make up RNA. The deoxyribose sugar is the sugar component of DNA. Each phosphate group connects the sugar rings of two adjacent nucleotide monomers. The phosphate groups and the sugar moieties form the backbone of a nucleic acid. The directionality of the chain runs from 5'-end to 3'-end. In DNA, the orientation of the two strands is in opposite directions. This is to allow complementary base pairing between nucleobase constituents. Apart from the long chain of nucleic acids, nucleotides also occur in cyclic forms. Cyclic nucleotides form when the phosphate group is linked twice to the sugar moiety, particularly to the two hydroxyl groups of the constituent sugar. The fundamental nucleotides are divided into purines and pyrimidines. In DNA, the purine bases are commonly adenine and guanine whereas the pyrimidine bases are typically thymine and cytosine. RNA includes adenine, guanine, cytosine, and uracil instead of thymine. The difference between uracil and thymine is the presence of methyl in thymine. While both purines and pyrimidines are heterocyclic aromatic compounds, they can be differed from each other based on the chemical structure. A purine has two carbon rings whereas a pyrimidine has one carbon ring. The purine has a pyrimidine ring fused to an imidazole ring. The pyrimidine has only a pyrimidine ring. Thus, the purine has four nitrogen atoms whereas the pyrimidine has two. Nucleotides may be grouped into mononucleotides, dinucleotides, oligonucleotides, and polynucleotides.


Characteristics

A polynucleotide is a compound comprised of several nucleotides (as opposed to oligosaccharides comprised of only a few, I.e. about three to twenty). Each monomeric component is comprised, in turn, of a nucleobase, a pentose moiety, and phosphate group. The monomers are joined together in a chain by ester linkages between the phosphoryl group of nucleotide and the hydroxyl group of the sugar component of the next nucleotide. Polynucleotides are one of the major biopolymers; the others are polysaccharides]] and polypeptides.


Examples

Deoxyribonucleic acid or DNA is a polynucleotide as it is made up of several monomeric units of nucleotides covalently bonded by 3', 5' phosphodiester linkages. This means that the 5'-phosphoric group of one nucleotide is esterified with the 3'-hydroxyl of the adjoining nucleotide. Each nucleotide, in turn, is comprised of phosphoric acid, a deoxyribose sugar (5-carbon), and a nitrogenous base. The nitrogenous base or nucleobase may be a cytosine [C], guanine [G],adenine [A] or thymine [T]. The two strands that make up the DNA form a helical structure wherein at the core the nucleobases are complementarily paired. The base pairing rules are adenine pairs with thymine whereas cytosine pairs with guanine. The bond that joins the two nucleobases is hydrogen bond. The two strands are antiparallel, which means they run in opposite directions to each other.


Ribonucleic acid (RNA) is a biomolecule consisting of a long linear chain of nucleotides. Each nucleotide unit is comprised of a ribose sugar, a phosphate group and a nitrogenous base (also called nucleobase). Ribose is a pentose sugar whose carbons are numbered 1' through 5'. The phosphate group is connected to the 3' of the ribose and the 5' of another ribose. The nitrogenous base is connected to the 1' of the ribose. The ribose sugars and the phosphate groups make up the backbone of the RNA. The nitrogenous bases of the RNA are, in general, adenine (A), cytosine (C), guanine (G), and uracil (U). Three major types of RNAs include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).


Biological importance

Nucleic acids, such as DNA and RNA, are natural polynucletoides. In particular, DNA codes for the sequence of amino acid during protein synthesis. It carries the genetic ‘blueprint’ since it contains the instructions or information (called genes) needed to construct cellular components like proteins and RNAs. In some viruses, RNA is the genetic material. For most organisms, RNAs are involved in: protein synthesis (e.g. mRNA, tRNA, rRNA, etc.), post-transcriptional modification or DNA replication (e.g. snRNA, snoRNA, etc.), and gene regulation (e.g. miRNA, siRNA, tasiRNA, etc.).


Polynucleotides are used in research and experiments. They are produced artificially from oligonucleotides for use in polymerase chain reaction (PCR) or in DNA sequencing.


Supplementary

Etymology

  • poly- ("many") + nucleo- ("nucleus") + -ide (chemical suffix)


Further reading

See also



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